Transition Metal Complexes with a Pentadentate Tetrapodal Phosphine Ligand for Synthetic Nitrogen Fixation : Synthetic, Spectroscopic and Mechanistic Investigations

This thesis deals in a first part with the coordination of the pentaPod ligand to various transition metal complexes. In the case of rhodium(III) and ruthenium(II), the electronic properties of the complexes and the influence of different small ligands, such as CN- and N3-, on these properties were investigated. This was achieved by using different spectroscopic methods (e.g. NMR, IR and Raman spectroscopy).

Furthermore, a tungsten(0) dinitrogen complex was synthesized based on the pentaPod ligand, corresponding to the already known molybdenum(0) dinitrogen complex. This could be realised by reacting the ligand with a suitable precursor complex and subsequent reduction of the resulting W(III) complex with sodium amalgam. The tungsten(0) dinitrogen complex was then investigated with regard to its ability to convert dinitrogen to ammonia. It was possible to generate 2.75 equivalents (equiv.) of ammonia, making the complex the first tungsten
complex capable of generating more than 2 equiv. of ammonia. The question arose as to why tungsten shows significantly less activity towards the catalytic conversion of dinitrogen to ammonia compared to the analogous molybdenum complex. Therefore, electrochemical and spectroelectrochemical investigations were carried out on both tungsten and molybdenum dinitrogen complexes bearing the pentaPod ligand. A significant difference in the stabilities of the complexes during oxidation to their respective M(I) species was found.

In addition to the experimental investigations, an extensive DFT study was carried out to gain detailed insights into the catalytic mechanism of N2 reduction using the pentaPod system and possible pathways of the competing hydrogen evolution during conversion into ammonia.


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